Parallel sha1

More work to speed up maketx.  My profiles show that computing the hash is becoming significant, especially when running maketx on a machine with many cores, because the SHA-1 hash is serialized.  SHA-1 is inherently serial, but... we can sha hash pieces of the image independently, then combine the results.

So this patch refactors computePixelHashSHA1 for parallelism:

1. Allow ROI to designate a section of the image.

2. Add an option to computePixelHashSHA1 to hash in blocks, then return hash of the hashes (versus hashing the entire image as one block).

3. When hashing in blocks, this can be multithreaded, since the computations are independent!

4. Change maketx to use the block-based hashing, in blocks of 256 scanlines. 

This will be incompatible with old maketx-generated hashes (both will be accepted by the texture system, but old will not match against new -- the hashes are different) but significanly speeds up maketx on multi-core machines.

I'm open to discussion/complaints about the compatibility issue.  I think that overall, we'll be happier with a much faster maketx that can be fully multithreaded (and, as well, anything else that makes use of computePixelHashSHA1).  But in the short term, it means that a texture made with the new maketx will not properly recognize itself as a duplicate of an identical texture made with the old maketx. (Old will still match against old, and new against new.) Is this an actual problem in the real world?  Probably only if you have LOTS of duplicate textures, split close to evenly between new and old maketx and you're unwilling to re-maketx the old textures to re-align the hashes.

My intuition is that this isn't a significant problem in a production environment, because as shows come and go, you will no longer use assets from old shows that were made with old maketx, and the new shows will use new maketx, and there will be very little mixture between the two (and if there is a small amount of mixing, who cares, it's not going to hurt too much and it'll be nice to speed up the maketx times).  But if you think I'm wrong about this, please let me know.